If Subjects Participating in a Two-Group Pretest-Posttest Design Research Study

Multiple Choice

If subjects participating in a two-group pretest-posttest design research study about bipolar disorder are given a knowledge test about the disorder being studied multiple times, what is the potential disadvantage?

A) The selection bias of only choosing subjects that take the test.
B) There will be an apparent lack of differences between the results
C) A threat to validity and mortality from repeated subject testing.
D) The longer time needed for testing requires a larger sample size.